Pen-down (shortened to pd) is used to trace the movements of turtles. The color of the pen is the same as the color of the turtle. While pen-down starts tracing, pen-up (shortened to pu) stops tracing a turtle’s movement. Think of pen-up and pen-down like a pen on the belly of a turtle; pen-down lowers the pen to the ground, leaving a mark of its path, while pen-up pulls the pen up, no longer marking the ground. Pen-down and pen-up are primitives used only with turtles. For example, ask turtles [ pen-down] would start drawing lines from the turtles’ movement, and ask turtles [ pen-up] would no longer trace the movement. (Pen-down, pen-up, and pen-erase are all equivalent to setting the turtle’s pen-mode to down, up, or erase)
The following code would make turtles draw squares and then move to a random location without leaving a trace:
ask turtles [
pen-down
repeat 4 [ forward 10 right 90 ]
pen-up
setxy random-xcor random-ycor
]
Pen-erase (shortened to pe) erases any previously drawn lines when the turtle passes over them. Think of it as an eraser on the belly of the turtle.
Once you mastered the pen-erase primitive, don't stop there. Check out the resources below to improve your NetLogo skills.